It is known that isocyanates can be prepared by reacting amines with phosgene. This reaction proceeds via the carbamic acid chloride, which decomposes at elevated temperatures into the corresponding isocyanate and hydrogen chloride. If the boiling point of the isocyanate to be prepared is distinctly higher than the decomposition temperature of the carbamic acid chloride, the hydrogen chloride liberated by the decomposition reaction can easily be removed from the reaction, especially if an inert organic solvent is used. If, however, the decomposition temperature of the carbamic acid chloride is close to the boiling point of the isocyanate or above it, the isocyanate enters the vapor phase above the reaction mixture and recombines with the hydrogen chloride to reform carbamic acid chloride. Decomposition is therefore incomplete in such cases and the isocyanate is obtained in only low yield and is contaminated with carbamic acid chloride.
These conditions apply to aliphatic monoisocyanates in which the aliphatic groups contain from 1 to 4 carbon atoms, the greatest difficulties being encountered in the preparation of methyl isocyanate.
Several processes intended to overcome these difficulties have been described in the patent literature. A major proportion of these processes involve the decomposition of carbamic acid chlorides with the use of hydrogen chloride acceptors.
Thus, for example, it is known to prepare isocyanates from carbamic acid chlorides in the presence of organic bases, e.g., tertiary amines, or carboxylic acid dialkylamides as described in German Offenlegungsschrift No. 1,593,554 or tetraalkyl ureas as described in U.S. Pat. No. 3,644,461 in organic solvents. The use of water as described in German Auslegeschrift No. 2,156,761 and of aqueous solutions or suspensions of inorganic bases as described in British Pat. No. 1,208,862 for the absorption of hydrogen chloride has also been described. Olefins have also been mentioned as hydrogen chloride acceptors in German Offenlegungsschrift No. 2,210,285.
All these processes have the serious disadvantage of giving rise to by-products, in the form of corrosive organic or inorganic salts or alkyl chlorides, which must either be removed by expensive methods or contaminate the surroundings. Moreover, the use of organic bases involves the risk of side reactions leading to dimers and trimers. In the presence of water, a considerable proportion of the carbamic acid chloride is hydrolyzed to the amide hydrochloride so that satisfactory yields can be obtained only in the case of the comparatively unreactive tertiary butyl isocyanate.
The preparation of low boiling aliphatic monoisocyanates by thermal decomposition of carbamic acid chlorides in organic solvents by special technical procedures is also known.
According to German Auslegeschrift No. 1,193,034, thermal decomposition of carbamic acid chloride is carried out in a reactor equipped with a reflux condenser and separating column. Hydrogen chloride escapes through the reflux condenser while the isocyanate, carbamic acid chloride and solvent are held back. The isocyanate formed in the reaction enters the column and can be removed at the head of the column. Most of the isocyanate is returned by means of a reflux divider so that the hydrogen chloride ascending the column is completely absorbed and returns to the reactor in the form of carbamic acid chloride.
When this process is carried out continuously, solution depleted of carbamic acid chloride is continuously removed from the reactor to be enriched with carbamic acid chloride in another apparatus and then returned to the reactor.
Variations of this process have been described in U.S. Pat. Nos. 3,969,389; 3,991,094; 3,969,388; and 4,069,238. These variations are based on the same principle as described above and differ only in the apparatus used.
Although the processes mentioned above make it possible for low boiling aliphatic monoisocyanates to be produced by thermal decomposition of carbamic acid chlorides, they have the following disadvantages:
1. The removal of hydrogen chloride requires reflux condensers with large cooling surfaces, which must be operated at high energy cost with a large amount of cooling fluid so that the isocyanate and carbamic acid chloride will be retained quantitatively.
2. Removal of isocyanate free from carbamic acid chloride by distillation from the reaction mixture requires highly efficient fractionating columns and adjustment of the reaction to a high reflux ratio.
3. Satisfactory results can only be obtained if relatively dilute carbamic acid chloride solutions are used (1 to 30%).
4. In a continuous process (which is the only kind suitable for large-scale commercial production), the reaction solution must be repeatedly recirculated.
All this means that the reactants (isocyanate, carbamic acid chloride and solvent) must be repeatedly evaporated, condensed or cooled and reheated during the process, which entails high energy consumption. The use of dilute solutions and the necessity for repeated circulation result in a long dwelling time and hence low volume/time yields. The long dwelling time involves the risk of reduction in yield due to trimerization of the monoisocyanate. The process requires elaborate measuring and control techniques. This, together with the low volume/time yields and the necessity of using highly efficient fractionating columns result in high investment costs for commercial production.